Image sensor having boosted reset

ABSTRACT

A power supply reset boosting element which boosts a level of the reset voltage to a level higher than the level of the power supply. The boosted voltage is isolated from both the power supply and from undesired switching by special transistors which can withstand the voltage power supply level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/917,195,filed Jul. 26, 2001, now U.S. Pat. No. 7,053,945, which claims benefitof U.S. Provisional application Ser. No. 60/221,347, filed Jul. 26,2000, which are hereby incorporated by reference in their entirities.

BACKGROUND OF THE INVENTION

CMOS active pixel sensors are known, and have been described, forexample, in U.S. Pat. No. 5,471,515. CMOS active pixel sensors are oftenvery small, in order to make them compatible with current elements.While the device includes an in-pixel follower and/or other in-pixeltransistors, size miniaturization often prevents a full complementaryMOS type switch. This means, therefore, that a full-scale reset functionwithin the active pixel circuit may be difficult.

In previous designs, this has prevented the operation from achievingfull signal reset. This has led to smaller signal swing, lowered signalto noise ratio, and a lag effect in adjacent frames due to theincomplete reset. This kind of reset is often referred to as a softreset. During such a soft reset, voltage levels on the sensor node maycontinuously change depending on the time that the reset persists.Ideally, this reset value should be set to the level Vdd−Vth, where Vddis the drain supply and Vth is the threshold voltage of the transistor.However, the actual value may vary.

The soft reset may also limit the effective signal swing. This can beespecially problematic in low voltage applications. For example, in aportable mode, the circuit may be powered from one of more batteries.Hence, the power supply may be limited to the battery voltage e.g., 1.5V or 3 V levels.

BRIEF SUMMARY OF THE INVENTION

The present application teaches a special circuit which may improvereset operation in an active pixel sensor type image sensor.

In one embodiment, a special bootstrap circuit is described which mayboost this voltage of the switch signal to a level above the availablepower supply and protect against this signal causing undesired effectselsewhere in the circuit.

Another embodiment describes a multi level reset circuit.

Yet another embodiment describes a circuit for use in a shared linereset system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described in detail, with referenceto the accompanying drawings, wherein:

FIG. 1 shows a bootstrap circuit of an embodiment;

FIG. 2 shows simulation results from the FIG. 1 circuit;

FIG. 3 shows an embodiment with improved switching and isolationcharacteristics;

FIG. 4 shows a simulation of the results of the FIG. 3 circuit;

FIG. 5 shows an improved boosting circuit with double level boosting;

FIG. 6 shows a row driver circuit;

FIG. 7 shows a row driver circuit with improved isolation; and

FIG. 8 shows a row driver circuit with separate boosting parts.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a “bootstrap” circuit of an embodiment. This bootstrapcircuit may increase the voltage output, e.g., the reset voltage level,to a level that is greater than the power supply voltage.

Driving signal 125 drives an inverter pair 115, 110. The inverter 110 isconnected to a first node 102 of the bootstrap capacitor 100. Theincoming signal causes the node 102 to initially charge to the negativesupply voltage Vss (e.g. ground) at node 102, while the other node 104of the capacitor 100 is held at the drain/supply voltage Vdd via PMOStransistor 120. The switching of the PMOS transistor is controlled bythe incoming signal on line 125.

Once node 104 is charged to the level Vdd, the boosting phase begins,when the input signal goes high, turning off transistor 120, therebyisolating the node 104 of capacitor 100, thereby floating the node 104of the capacitor. During the same clock cycle, and two inverter delayslater, the signal at 102 is brought high, thereby boosting the voltageat node 102. In principle, the voltage at node 104 should be capable ofbeing pumpable to a voltage close to double the level of Vdd. This valuemay be used as a pixel reset value.

In reality, a steady reset level will not be obtained, but rathersimulation results are as shown in FIG. 2. For a 3.3 volt power supply,the node 102 may be pumped to a voltage around 5 V initially, butreaches a voltage less than 4 V in the steady state. Also, the level ofthe pulse varies according to a “pulse” shape, making it less suitablefor use as a reset pulse.

The present inventor believes that the PMOS switch 120 was causing theseproblems, for the following reason. The PMOS transistor 120 is used inthe circuit of FIG. 1 to precharge the capacitor 100 during the clocklow phase. During the boosting phase, the switch 120 is turned off toisolate node 104. The inventor believes that transistor 120 does a verygood job of precharging the capacitor. However, the relatively highvoltage that occurs as a result of the pumping may effect thetransistor's ability to isolate.

When a “normal” reset signal is being produced, Vdd as applied to thePMOS gate can effectively isolate the drain node from the source node.The voltage at node 104, however, may be as large as double the level ofVdd.

The results are that during the initial portion of the boosting phase,the voltage rises, causing the PMOS gate to conduct between node 104 andthe power supply even if the gate voltage of the PMOS is at Vdd. Thiscauses charge from the node 104 to conduct to the power supply line,reducing the pulse voltage. This causes the spike shaped waveform thatis noticed.

In addition, a spike of this shape may be injected on to the powersupply line. Moreover, since this power supply may be connected to otherpower supply lines, noise may be added to the entire circuit that isbeing powered by these lines. This in turn may degrade the performanceof the chip.

Moreover, the boosting circuit has typically been shared between anumber of different rows of an image sensor. The passing gate from theboost line to each row reset line may also be implemented by a PMOSswitch, to allow the relatively high voltage to be transferred. This mayaffect other rows in addition to the current row being reset. This maygenerate sufficient voltage on all reset lines, through their respectivePMOS pass transistor, even though the gates of those transistors areheld high. Even with pulldown resistances on each reset line, theinjection of the boosting signal may produce a pulse e.g. as high as 500mv. This pulse may cause difficulty in obtaining a reliable high qualityimage.

Other boot strap circuits are known, including the one described by T.B. Cho and P. R. Gary, “A 10-bit, 20 Msample/s, 35 mW Pipelined A/Dconverter” IEEE J. Solid-State Circuits, Vol. 30, pp. 166–172, March1995.

FIG. 3 shows another embodiment to provide a boot strapping circuit toproduce a hard reset signal that may avoid many of the issues notedabove. In order to avoid the problems noted above, a special transistorpair is used for precharging. The precharge transistor 300 has a firstnode connected to reference voltage 305. For example, the drain oftransistor 300 may be tied to the reference voltage 305. This referencevoltage will be boosted to a level above the supply voltage Vdd.

The initial precharge phase begins when the input signal 350 is low. Thesignal is inverted by inverter 354 to a high level, and drives thetransistor 300 to conduct the reference voltage 305. Two more invertersin a line also drive the transistor 360 which is a PMOS transistor. Thehigh level at the gate of the PMOS 360 turns that transistor off, thusus allowing Vcc to turn on the transistor 302. Thus, the voltagereference 305 is conducted to the rear plate node 314. The voltage atthe output of inverter 354, again a high level, also drives NMOStransistor 356, turning on that transistor, and thereby holding thefront plate 312 of capacitor 310 at ground level. Thus, during thisinitial stage, the capacitor 310 has its front plate 312 being grounded,while its rear plate side node 314 is charged to the level of thereference voltage 305 through the precharge transistors 300, 302.

A transfer gate 320 is also held at Vdd in order to isolate the boostingline from the output node 330.

At the time of the initial precharging, the output node 330 is pulled toground via applying a high level to the gates of output pulldowntransistors 340, 342.

During the boosting phase, the input signal at node 350 goes high, thesystem operates by first turning off the transistor 352 initially. Afterone further inverter delay by inverter 354, the signal turns off thetransistors 340, 342 removing the ground level on the output node 300,thereby removing the Vref charge and floating the node 314 of thecapacitor, and transistor 356, thereby removing the ground on thecapacitor node 312. The precharge transistor 300 may be an N typetransistor. Therefore, when its gate voltage is lowered by the output ofinverter 354, a substantially perfect block against the boosting line,and the reference source Vref, may be obtained.

The circuit then may turn on a transfer gate 320, which allows any highvoltage above the threshold to be passed to the output node.

Finally, Vcc is applied to the front plate of the capacitor by turningon the charging transistor 360. This provides a substantially perfectflat boosting voltage on both the boosting line and on the output node.This prevents charge leakage both during the boosting phase and duringswitching.

The system has another advantage that it does not require dynamicprecondition to be set prior to a first execution. Therefore, thiscircuit can be used for both continuous clock operation as well as withsingle pulse application.

Ideally, this system can boost a voltage of as high as Vdd+Vref.However, the charge redistribution over capacitor 310 and the othercapacitors connected to the boosting line may redistribute the charge.This means that the practical value of the boosting voltage will be

$V_{boost} = {V_{dd} + {\frac{C_{1}}{C_{1} + C_{parasitie}}V_{rcf}}}$

This implies that driving the capacitor load may be maximized when theboosting capacitor is large enough to compensate for this reduction.

FIG. 4 shows a simulation of both a single reset pulse in the uppertrace, and continuous power reset pulses in the lower trace.

In certain applications, two separate boosted values may be useful,e.g., to boost the level of a local n well at the same time as theboosting the reset level.

FIG. 5 shows a double boosting circuit which forms a second embodiment.The circuit produces two different boosted outputs, including a levelcontrolled boosting signal on the output line 500, which may be forexample Vdd+2 volts in a 3.3 V system. A second output 510 produces ahigher level voltage boosting such as for a local n well boosting. Thissecond level may be closer to 2×Vdd.

As in the FIG. 3 embodiment, the FIG. 5 embodiment also includes a lineof inverters, here shown as 502, 504, 506. The signals from the signalchain, as inverted by different inverters, respectively drive twodifferent sides of first and second capacitors 520, 522. The firstoutput 500 is formed from the output of capacitor 522. One end of thiscapacitor is driven from the reference voltage 524 through a pair ofswitching transistors 526, 528, much like in the first embodiment. Also,as in the first embodiment, the capacitor 522 can have its first plateshorted to ground via transistor 533, or driven with a voltage Vcc viatransistor 535. The difference between the two voltages, that isVcc+Vref may be present on the output line 500, as switched through thetransistor 531.

That same transistor 535 that produces the voltage to capacitor 522, mayalso produce the voltage to the second capacitor 520. This transistormay be precharged with a different value e.g., it may be charged withVcc through one of the transistors 536. The output which it produces onthe output line 510 may therefore be close to 2×Vcc.

This system has special parts which may avoid breakdown, punch throughor latch up in the IC. In this system, the body of the transfer gate maybe tied to the boosting line which may prevent latch up. The othertransistors that are associated with the high voltage node are notsimilarly controlled. However, by adding additional transistors 528 and537, at any moment the source drain voltage is insured not to exceed onepower supply voltage. This may prevent the circuit from breaking down orpunching through.

This system may improve the signal produced during the reset operation.

In another embodiment, this circuit may be applied to a linear sensorarray using a row driver. In many of these systems, the line is sharedbetween normal row reset and boosted row reset. In the multirow sensorarray, the standard row driver would encounter several problems. Thismay be especially problematic when a shared line is used to implementthe normal row reset and boosted reset operation.

Another embodiment describes a new row driver that enables sharing rowselection and high voltage boosted reset. This circuit may carry outregular row selection during one mode and boost the same voltage line upto a controllable level during a reset operation. This may preventcharge leakage from the boosting line to the rest of the circuitrythrough the transistors. Therefore, this system may also eliminate noisespikes used by the boosting.

A row driver is shown in FIG. 6. Each row line is used as a reset lineduring an idle period in order to reset the function in the currentlyselected row. However, this may have the same problem as those discussedabove when the reset level is increased. For example, the increasedreset level may forward bias one of the switching transistors 600, 602,604. For example, this may degrade the reset voltage, by causing thespike shaped pulse shape described above. It may also pass this resetvoltage onto the row bus and to the power supply, as described above.

When the line is being used as a reset line, the transistors 600, 602are shut down by applying Vdd to their gates. However, since the boostedvoltage is above Vdd plus Vthp, where Vthp is the threshold voltage ofthe PMOS transistor, both transistors may actually fully conduct. Thismay lead to two separate problems. The first, the boosting charge mayquickly leak through the two transistors 600, 602 to the power line.This prevents the high voltage reset from being completely implemented.Second, if the boosting capacitor is sufficiently large, which may bethe case as described above, this may generate significant noise spikesin the power supply line.

During times when the row is not selected, additional problems mayoccur. The row signal is intended to be low, and isolated from theboosting signal. However, when the boost is increased enough, thisboosting signal may pass onto each of the row biases; even those thatare not currently selected. Ideally the transistor 604 is turned off byapplying Vdd to its gate. However, when the boosting voltage is aboveVdd plus Vthp, the boosting signal may be transferred onto all the rowselect lines. This may turn on multiple rows and also act as a weakreset applied to each pixel. This could damage the pixel array leadingto signal degradation.

A new system that avoids this problem is shown in FIG. 7. This systemallows both sharing the row line between reset and row select, the alsoallows a high (boosted) voltage reset. The system shown in FIG. 7 uses alatch type toggle structure to form selected levels for the boostingline and for the row select operation.

In this embodiment, each of the row lines is used as the reset line whenthe previous row is selected for readout. Therefore, when the previousrow is selected, and the current row is in standby mode, the output ofthe current row is set to ground or to a given reset voltage. Anexternal reset signal is applied into the driver block, forming a lowsignal to be sent to this driver on the FROM PREVIOUS input 702. Thissignal being low turns off the switching transistors 724, therebyproducing a low output at 716. This low output is conducted to the gateof transistor 710. This turns off the transistor 710, and thus blocksthe output from the reset low node. Subsequently, the system turns offthe transistors 712 and 714 by pulling the node 717 to ground via thetransistor 725. This also links the boosting line to the output.

The voltage on the output is a balanced voltage set by Vdd which islocated at the source of the transistor 716 and the precharge voltage727 from the bootstrap circuit. This voltage enables the pixel to stayin the Preset mode e.g., half reset. When the boosted signal arrives atthe node 727, it raises the voltage at node 735 and hence completelyturns off the transistors 716 and 718 since the voltage on the node 735is at the boosting level. Since the gates of these transistors 718 and716 are at this boosted level, these two transistors are now able tosubstantially perfectly isolate the boosting line 727 from any of theother circuitry lines including from Vdd. Thus, no leakage signal passesonto the power supply or to the other portions of the circuit. Only theoutput line is boosted to a high voltage on the selected row.

When the reset value goes low, the output line is pulled down again toresume the ground state or reset low voltage. When the current line isselected, the signal from FROM PREVIOUS remains high all the time,turning on the transistors 717 and 716 and allowing Vdd to pass to theoutput of the driver. During the time that bootstrapping takes place,the voltage on the boosting line will be increased to a higher resetvoltage. At the same time, it completely isolates the row output line byapplying the same voltage to the gate of the transistors 717, 716. Thisprovides an improved environment for row select operation.

This row driver array may be implemented at a 5.5 micron pitch, forexample.

The embodiment of FIG. 8 shows an alternate embodiment of which may haveseparate boosting capabilities. In the FIG. 7 embodiment, the prechargelevel of the boosting circuit is presumed to be Vcc. However, a levelcontrolled bootstrap function may encounter charge leakage through thesilicon body.

In the embodiment shown in FIG. 8, a separate boosting line 800 isprovided. The bodies of a plurality of switching transistors 805, 810,815 are connected to this dedicated boosting line. Using a boostingsystem as shown in FIG. 5, allows the boost line 100 to be used as thenormal bootstrap circuit at the desired boosting level. A separate boostline 820 may be connected, and may be used to bring up the local bodypotential of certain transistors. This ensures that the local bodypotential will always be higher than the source line and the drain lineof the switching transistors, even when a boosting voltage is applied.

Although only a few embodiments have been disclosed in detail above,other modifications are possible. All such modifications are intended tobe encompassed within the following claims, in which:

1. An image sensor, comprising: a photosensitive region; a voltageboosting circuit for producing a boosted reset voltage on a reset lineat a level higher than a power supply output voltage level, said voltageboosting circuit comprising a first capacitor, a precharge circuit forprecharging said first capacitor and an isolating circuit for isolatingsaid capacitor, said isolating circuit is not forward biased by avoltage that is greater than the power supply voltage, wherein a chargeon said first capacitor is used as said boosted reset voltage; and avoltage protection circuit, said voltage protection circuit beingconnected to said reset line and protecting at least one transistoragainst being forward biased by said boosted reset voltage.
 2. The imagesensor according to claim 1, wherein said voltage protection circuitfurther comprises at least one additional transistor of a type which cannot be forward biased by said boosted reset voltage.
 3. The image sensoraccording to claim 1, wherein said isolating circuit comprises first andsecond series connected switching transistors.
 4. The image sensoraccording to claim 1 further comprising a second capacitor, wherein acharge on said second capacitor is used as a second boosted output. 5.The image sensor according to claim 1 further comprising a row drivercircuit, which uses a first line as a row select, and a second line,intended for row select of a different row than a currently selectedrow, for a reset line.
 6. The image sensor according to claim 1, whereinsaid precharge circuit comprises a switching transistor.
 7. The imagesensor according to claim 1, wherein said boosting circuit furthercomprises a series of inverters used to generate control signals forsaid boosting circuit.
 8. An image sensor comprising: a photosensitiveregion; and a voltage boosting circuit for producing a first boostedvoltage on a first line and a second boosted voltage on a second line,said first voltage being at a first level higher than a power supplyoutput voltage level, said second voltage being at a second level higherthan the power supply output voltage level, said voltage boostingcircuit comprising: a first capacitor, a second capacitor, a prechargecircuit for precharging said first and second capacitors, and anisolating circuit for isolating said capacitors during operation of saidboosting circuit, said isolating circuit is not forward biased by avoltage that is greater than the power supply voltage, wherein a chargeon said first capacitor is used as said first boosted voltage and acharge on said second capacitor is used as said second boosted voltage.9. The image sensor according to claim 8, further comprising a voltageprotection circuit, said voltage protection circuit being connected toat least said first line to protect at least one transistor againstbeing forward biased by said first boosted voltage.
 10. The image sensoraccording to claim 9, wherein said voltage protection circuit is alsoconnected to said second line to protect at least another transistoragainst being forward biased by said second boosted voltage.
 11. Theimage sensor according to claim 8, wherein said first line is a resetline and said first boosted voltage is a boosted reset voltage.
 12. Theimage sensor according to claim 8, wherein said first line is a resetline, said first boosted voltage is a boosted reset voltage, said secondline is a reset well line and said second boosted voltage is a boostedreset well voltage.
 13. The image sensor according to claim 8, whereinsaid boosting circuit further comprises a series of inverters used togenerate control signals for said boosting circuit.
 14. An image sensor,comprising: a voltage boosting circuit for producing a first boostedvoltage on a first line, said first boosted voltage having a levelhigher than a power supply output voltage level, said voltage boostingcircuit comprising: a first capacitor, a precharge circuit forprecharging said first capacitor, and an isolating circuit for isolatingsaid capacitor, said isolating circuit is not forward biased by avoltage that is greater than the power supply voltage, wherein a chargeon said first capacitor is used as said first boosted voltage; and avoltage protection circuit, said voltage protection circuit beingconnected to said first line and protecting at least one transistoragainst being forward biased by said first boosted voltage.
 15. Theimage sensor of claim 14 further comprising a second capacitor, whereina charge on said second capacitor is used as a second boosted voltage.